The invention relates to an arrangement for the determination of the effective pulmonary blood flow.
From the paper Steinhart, C. M., Burch, K. D., Bruno, S., Parker, D. H.: Noninvasive determination of effective (nonshunted) pulmonary blood flow in normal and injured lungs, Crit. Care Med., 1989, Vol. 17, No. 4, pp. 349-353 the Multiple-Inert-Gas method with rebreathing of helium, acetylene and carbon monoxide in oxygen and nitrogen from a respiratory bag is known. A disadvantage of this measurement is the relatively great effort to prepare the gas mixtures for rebreathing, the demand for special measuring instruments that can measure the concentrations of gases in the breathed air or the breathing flow, respectively, and the necessity of a person to connect the respiratory bag to the patient and maintain it. For those reasons the measurement of the effective pulmonary blood flow can only be executed by specialists and is for research purposes in almost all cases.
Further, from the paper by Inman, M. D., Hughson, R. L., and Jones, N. L.: Comparison of cardiac output during exercise by single-breath and CO2 rebreathing methods, J. Appl. Physiol., Vol. 58, pp. 1372-1377, 1985, the total CO2 rebreathing method and the so-called single-breath method are known. Disadvantages of these methods are distinct increases of CO2 pressure in the arterial blood, as the CO2 elimination is interrupted, and the impeding of the respiration. Other disadvantages are those of the Multiple-Inert-Gas method, namely the preparation of gas mixtures for rebreathing having certain CO2 concentrations, the demand for special measuring instruments that can measure the concentrations of gases in the breathed air or the breathing flow, respectively, and the necessity of a person to connect the respiratory bag to the patient and maintain it.
Another method known is a partial CO2 rebreathing method (according to Gedeon, A., Forslund, L., Hedenstierna, G. and Romano, E.: a new method for noninvasive bedside determination of pulmonary blood flow, Med. & Biol. Eng. & Comp., 1980, Vol. 18, pp. 411-418) based on varying minute ventilation. Disadvantage of this method is the variation of the mean respiratory tract pressure and of the pressure at the end of expiration. Due to this variation of the respiratory tract pressure the pulmonary blood flow also varies and both the mechanical stability of the lungs and the gas exchange are impeded.
From the paper by Capek, J. M. and Roy, R. J.: Noninvasive measurement of cardiac output using partial CO2 rebreathing; IEEE Transactions on Biomedical Engineering, 1988, Vol. 35, No. 9, pp. 653-661) the partial CO2 rebreathing method with change of the dead space of the apparatus is known, which is performed using a mass spectrometer and special respiratory flow sensors at the endotracheal tube and can measure the total cardiac output. The change between two different dead spaces is performed by a PC-controlled electromagnetic valve. The partial expiration termination pressure and the CO2 elimination are determined for the respiration by both dead spaces. The CO2 partial pressure at the end of expiration is converted to the arterial CO2 concentration and the total cardiac output can be calculated from the division of the CO2 elimination difference by the arterial CO2 concentration difference. A disadvantage of this method is the demand for apparatus for the measurement of the CO2 concentration in the breathed air (mass spectrometer) and of the respiratory flow at the endotracheal tube (Fleisch pneumatocograph).
It is the objective of this invention to describe a clinically practicable arrangement for noninvasive determination of the effective pulmonary blood flow whereby the effective pulmonary blood flow is the cardiac output minus the intrapulmonary shunt proportion. Only that portion of the cardiac output is intended to be determined which is available for gas exchange. This arrangement requires only little apparatus, does not essentially influence the respiratory schedule, and is capable of being automated.